If lumber is not properly equalized and conditioned during drying, problems will surface during further manufacturing and end use. These problems range from bad glue joints, to stair stepping1 in panels, to twist, cup and warp of finished panels, to splitting of panels.
1 Stair stepping is seen in finished edged glue panels that contained some parts that were either too wet or too dry when assembled that subsequently shrunk or swelled in thickness when the panel equalized to atmospheric conditions or the panel contains both flat and quartersawn material that differentially shrunk or swelled when the panel equalized to atmospheric conditions.
Decide on Your Target Moisture Content
It is critical that a kiln operator have a concrete idea what their target moisture content is for a kiln charge. What the operator does during the drying process and at what equilibrium moisture content (EMC) they use to equalize and condition depends upon the target moisture content. The target moisture content should be based on the EMC of the manufacturing environment and the EMC of the end use.
The conditions in the furniture plant should be controlled as much as possible to emulate the environment the product will be in during its end use. The target moisture content may be lower in the winter (6 percent moisture content) and higher during the summer (8 percent moisture content).
The purpose of equalizing is to group the moisture content of all of the boards in a kiln charge around the desired target moisture content. We would like to see a very tight or close distribution of the moisture contents in a kiln charge around the target.
Boards that are too wet can cause gluing problems, warpage in finished panels or stair stepping in panels. Boards that are too dry can cause similar problems as well as machining problems. In addition to the above problems, lumber that has not been properly equalized cannot be properly or uniformly conditioned! The purpose of conditioning is to add a prescribed amount of moisture to the outside shell of a board to relieve drying stresses. If a board is too dry prior to conditioning, too much moisture will be added and reverse case hardening will result. If a board is too wet prior to conditioning, too little moisture will be added and the lumber will not be conditioned.
Non-uniformity in the moisture content of a kiln charge of lumber can sometimes be attributed to the different drying rate of the boards located on the outside edge of a kiln charge versus those boards located in the center pack of the kiln charge. This is due to the fact that as air moves across a pack of green lumber, it drops in temperature, losing its potential to dry lumber. The difference in the drying rate between the center of the charge and the edge boards increases with higher starting moisture contents, lower air velocities, and longer lengths that the air has to travel before reheating. When drying green lumber at lower air speeds and small initial wet bulb depressions, the air is fully saturated several feet into the outside package. This means no drying occurs in the center package until the outside edges have had a good head start in drying.
When drying species that are prone to collapse or honeycombing (especially quartersawn lumber) it is critical to control the raising of the dry bulb temperature. This should be based on the moisture content of the interior pieces of lumber, not on the moisture content of sample boards located on the outside edge of the kiln charge.
Equalizing is used to obtain a uniform final moisture content. Using an equalizing EMC of the target moisture content minus two (target MC% – 2) will insure against equalizing too low which will prevent reverse case hardening versus using an EMC of the target moisture content minus three (target MC% – 3).
By never having an EMC below the equalizing EMC (target MC% – 2) the operator will be assured of not over drying lumber, decreasing warp in the lumber, and also insuring a more uniform moisture content and better results in conditioning. By equalizing during the entire final stages of drying at an EMC of the target MC% – 2, the kiln schedule will be lengthened slightly, but the results in terms of quality should pay off. If drying needs to be sped up during the final stages, an EMC of the target moisture content minus three (target MC% – 3) can be used. However the EMC should be raised to the target moisture content minus two (target MC% – 2) when the driest boards in the kiln charge reach the target moisture content minus two (target MC% – 2).
How long should one equalize? The drying manual says until the wettest sample reaches the target moisture content. In actuality we want to equalize slightly longer especially if we have a large difference in the drying rate between the outside edge and center of the charge. Two ways of evaluating this are: 1) using sample boards; matched samples should be made of the wetter and slower drying samples and placed in the center of the kiln charge for evaluation at the end of the kiln run or 2) meter the center pack and compare the moisture content of the center pack to the samples.
The purpose of conditioning is to relieve drying stresses commonly known as case hardening. This is done by adding moisture to the outside shell of a board, allowing the wood fibers to creep or, in other words, relax. Conditioning is accomplished by raising the EMC inside the kiln so that the lumber’s surface will gain moisture. For softwoods the rule of thumb for conditioning is to set the EMC inside the kiln to the target moisture content plus three (target MC% + 3). For hardwoods the rule of thumb for conditioning is to set the EMC inside the kiln to the target moisture content plus four (target MC% + 4).
A very important concept is that for conditioning to actually occur the EMC inside the kiln must actually rise to the desired conditioning EMC. One of the first places to look for problems in conditioning is to look at the actual kiln conditions and determine if you have the desired EMC. In some kilns the dry bulb temperature will increase when the wet bulb is raised. This dry bulb temperature override will result in a lower EMC than that required for conditioning.
Two possible solutions to a rising dry bulb temperature as the wet bulb is raised are to: 1) cool the kiln down before starting conditioning or 2) use a water mist spray to condition. By cooling the kiln down prior to conditioning, some of the energy released by the steam and the steam and wood interacting will be used to heat up the kiln. After the kiln is turned back on, the dry bulb is left shut off and the wet bulb is raised to the desired wet bulb for conditioning. The operator needs to observe the kiln controller and see what the actual dry and wet bulb temperatures are inside the kiln, and then evaluate if the kiln is at the desired EMC for conditioning by looking at an EMC table.
The reasoning behind a water mist conditioning system is that for much of the energy created during conditioning, the energy trade-offs for using a water mist system are a zero sum gain. The mist system has been used in many European kilns and has gained in popularity recently in the United States.
Traditionally conditioning has been evaluated using the prong test. The operator has been told to continue conditioning until done. To get good readings from a prong test one should wait several hours prior to evaluating the prongs. This delay does not do any good for production purposes. A quicker way to evaluate conditioning is to see if the lumber has picked up approximately 1 ½ percent moisture content. If the sample is at six percent at the start of conditioning, it should be at 71⁄2 percent at the end of conditioning. Thus the operator needs to weigh the samples prior to conditioning and immediately after conditioning.
A problem seen in some plants is that the operator or the rough mill is cutting their prongs too thin often giving false readings. A good rule of thumb for 4/4 and 5/4 lumber is to cut the prongs one-fourth the thickness of the lumber. Also another good evaluation tool is to run a sawline down the center of the piece.
For lumber that is going to be used for long rips that are required to be straight longitudinal stress samples are in order. These longitudinal stress samples closely approximate the resulting strips seen coming out of the rip saw.
Publication date: May 1, 1997
N.C. Cooperative Extension prohibits discrimination and harassment regardless of age, color, disability, family and marital status, gender identity, national origin, political beliefs, race, religion, sex (including pregnancy), sexual orientation and veteran status.